A macro processor is a program that reads a file (or files) and scans them for certain keywords. When a keyword is found, it is replaced by some text. The keyword/text combination is called a macro.
A simple example is the C language preprocessor. When you write
#define MAX_BANANAS 6
int banana;
for (banana = 0; banana < MAX_BANANAS; banana++){
...;
}
in a C program, the C preprocessor reads the first line and stores it as a macro definition. When it comes across the later reference to MAX_BANANAS in the for loop, it replaces it with the macro's definition, 6. The output of the C preprocessor is then fed to the C compiler proper.
M4 and the htm4l macros work in a very similar way. For example, the htm4l macros allow me to write
in a .htm4l file, which produces
when run through m4. This is a very simple example. The htm4l macros get much more complicated (and useful!).